Fewer MPs' nominations would be needed for the next leader.

Fresh plans to change Labour’s leadership rules to ensure a left-wing successor to Jeremy Corbyn are set to be debated by the party’s annual conference, HuffPost UK has learned.

A motion from the TSSA train workers’ union is to be put before the party’s ruling National Executive Committee (NEC) this month, with the aim of watering down MPs’ power of veto over future candidates.

Under the new plan, seen as a “compromise” that will get majority backing, the current requirement for nominations from 15% of MPs and MEPs will be reduced to just 10%.

At present, 42 Parliamentarians are needed for any new leadership candidate, but if the change gets the go ahead, just 28 nominations will be required.

The move to lower the threshold is seen as essential by some on the Left as vital to ensuring a leftwinger can succeed Corbyn to carry on his radical policy agenda once he steps aside.

Corbyn narrowly secured the 35 MP nominations he needed in 2015, before he went on to win a landslide in the leadership election.

PA Archive/PA ImagesTSSA union leader Manuel Cortes.

The TSSA union - which is a strong supporter of Corbyn - will move the motion at a crunch meeting of the NEC, paving the way for a vote on the rule change at the party conference in Brighton.

General Secretary, Manuel Cortes told HuffPost UK: “TSSA have submitted a rule amendment for Conference over the election of the leader of the Labour Party.

“We are proposing that those wanting to stand must be nominated by at least 10% of the Parliamentary Labour Party and European Parliamentary Labour Party.

“Like any Conference business, this matter will be discussed by Labour’s National Executive Council prior to Conference.”

The amendment was sent to Labour HQ earlier this summer in June and is now expected to be moved at the key NEC meeting ahead of conference. The union has ‘played by the rules’ and wants to make sure the Left is ‘seen to be on the side of democracy’, insiders said.

The TSSA is expected win the backing of Unite and other key trade unions, as well as constituency Labour parties and the increasingly powerful Momentum movement.

The swing vote of the GMB union is still unclear, but some insiders claim it won’t oppose the plan when it comes before conference.

Backers of the idea say it is part of a wider move under Corbyn to democratise the party and give members more of say by ensuring as wide a selection of candidates as possible.

The move is a bid to provide an alternative to a plan to cut the percentage of MP and MEP nominations to 5%, seen as a step too far by some ‘centrists’ and trade unions.

Moderates believe the shift came because unions were unconvinced by the 5% plan and faced defeat at conference. Party insiders have pointed out that no compromise was possible without a formal motion circulated to the NEC, but that is now set to change after the TSSA move.

ProgressHow Progress depicts the rule changes.

Blairite group Progress has dubbed the 5% idea “the McDonnell amendment” - a term he firmly rejects as “propaganda” - because the Shadow Chancellor stood for leader twice before, and has campaigned vigorously against it.

Progress chief Richard Angell has claimed that the 5% figure would be a “pitiful threshold”.

“Not only is ‘Clause One’ of the party rule book to ‘maintain in parliament and the country a political Labour party’ but our system requires the candidate to be prime minister to command overwhelming support on the treasury benches. The hard-left’s amendment acknowledges that their candidate for leader will never command that kind of support.”

Progress points out that the threshold was 12.5% when MPs were part of an electoral college that elected the leader. A proposal to increase that to 20% was rejected by Ed Miliband in favour of 15%.

Some senior figures think that the whole idea is now academic as a new influx of young pro-Corbyn MPs makes it easier to win much more than the handful of nominations that left-wingers used to command in the pre-Corbyn years.

While some unions see the rule change as a distraction from the fight against the Tories, others believe the 10% compromise will allow the party conference to unite and focus on other issues.

PA Archive/PA ImagesJeremy Corbyn after an NEC meeting that guaranteed his leadership candidacy.

Members of the NEC have told HuffPost UK that the new plan is likely to be approved for debate by the full conference. “It’s a decent compromise,” one source said.

A rival plan, to change the system to allow unions and party members to nominate leadership candidates, is seen as too complex, as well as too confrontational for the Parliamentary Labour Party (PLP).

With the Left of the party increasingly confident, ‘centrists’ in the Progress and Labour First groupings are holding a series of “Moderate Meet-Ups” with members and MPs in the run-up to the Brighton gathering.

The first event took place on Thursday night in Sheffield, with CLP delegates from the local area attending. It will be followed by others in London, Sunderland, Cardiff, North Wales, Birmingham and Edinburgh.